The present invention relates generally to active cancellation systems for repetitive phenomena, and more specifically to a selective active cancellation system in which the cancellation takes place within an electronic circuit, i.e., in-wire.
Prior art analog notch filters are difficult to tune as the frequencies to be rejected change, require tight tolerances of components and often require non-standard values for components to select precisely the frequency to be rejected and have a small bandwidth.
Prior art active cancellation systems have required a sensor, such as a microphone, to detect the noise and an actuator, such as a speaker, to produce the cancelling antinoise. The present invention is directed toward eliminating the sensor and actuator for the cancellation of repetitive noise within communications, surveillance and related systems.
Linear flow, air duct systems, for example, Chaplin U.S. Pat. No. 4,122,303; Warnaka U.S. Pat. No. 4,473,906 and Eriksson U.S. Pat. Nos. 4,677,676 and 4,677,677, take advantage of directional flow in linear, one dimensional flow to utilize an upstream sensor, followed by a cancellation actuator and downstream error sensor in sequence. These systems cancel repetitive and random noise. Chaplin characterizes the control as a general convolution process, including a "programme of time-related operational steps." Warnaka uses adaptive filters to speed adaptation time and allow greater spacing between the speaker and the duct. Erikkson specifies recursive least means square (RLMS) and least means square (LMS) adaptive filters to perform the convolutions and measure the system transfer function sin the presence of noise.
These systems require both an upstream sensor and a downstream sensor and an actuator.
Systems for canceling repetitive noise and vibration, for example, Chaplin U.S. Pat. Nos. 4,153,815 and 4,417,098, describe the use of a synchronizing timing signal to provide selective cancellation of repetitive noise or vibration. Additionally a controller, actuator and error sensor are used. The method presented by Chaplin in these patents divides the noise or vibration period into a number of intervals and adjusts the amplitude of the canceling signal within each interval in response to the sign or amplitude of the error sensor within the same or a delayed interval.
In U.S. Pat. No. 4,490,841, Chaplin describes the use of Fourier transforms to process signals in the frequency domain. While this method might be used for random signals, processing time requirements generally limit its application to repetitive signals.
These systems require an error sensor and an actuator.
Cancellation of unwanted components within electronic signals generally is applied to communication signals. Renneck et al. in U.S. Pat. No. 4,232,381, use a commutation filter synchronized to the rotation of an engine to cancel self-generated engine noise within an electronic circuit. The level of the canceling signal is adjusted manually and no method is provided to adapt to phase shifts or varying amplitudes of different harmonics.
Garconnat et al. in U.S. Pat. No. 4,594,694 use two sensors, one sensing both the wanted and unwanted signals and the other sensing only the unwanted signals. Narrow band filters or Fourier transforms are used to eliminate the unwanted signals from the combined signal.
Widrow in "Adaptive Noise Canceling Principles and Applications", Proceedings of IEEE, Vol. 63, No. 12, December, 1975, describe two forms of active adaptive cancelers. The first, as illustrated in FIG. 1, uses a multi-tap adaptive FIR filter with a reference signal correlated with the noise to be cancelled. The reference signal is required to be within 90.degree. in phase of the error signal. Consequently, the reference signal used by the adapter itself often requires filtering; the resulting approach is referred to as the "filtered-x algorithm."
The second form described by Widrow, as illustrated in FIG. 2, provides a single frequency notch filter and requires only two single tap filters. Again, a reference signal correlated with the noise is used and is phase shifted 90.degree. for one of the filters. Glover, in "Adaptive Noise Canceling of Sinusoidal Interferences," Stanford University, Stanford, CA, May 1975, Ph.D. dissertation, extended this technique to multiple frequencies.
Each of Widrow's techniques requires a sensor for the reference signal, a sensor for the primary signal and a sensor for the error signal and requires that the cancellation take place numerically within the processor. Thus, the processor and associated elements must have sufficient bandwidth for all frequency components of the signal, not just those components to be cancelled.
Thus it is an object of the present invention to provide a selective active cancellation system for repetitive phenomean that eliminates sensors for the noise and the error and eliminates the actuator.
These and other objects are obtained by providing an electronic selective active cancellation controller and electronic mixer circuitry with inputs for the primary signal containing both noise and intelligence and for the cancellation signal from the controller and an output of the signal containing the intelligence with the repetitive noise cancelled.
In preferred embodiments, the cancellation controller contains all necessary filters, including antialiassing filters and reconstruction filters. Since the primary signal containing both the noise and intelligence is not passed through the cancellation controller, the intelligence signal component is not degraded by the controller filters and other circuitry. The primary signal is thus only affected by the repetitive noise cancelling signal fed to the electric mixer.